Magnetic resonance imaging (“MRI”) is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.
MRI uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During an MRI procedure, the patient is inserted into an imaging volume of a primary field magnet. The vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons tends to align with the direction of the static magnetic field generated in the imaging volume by the magnet. A transmitting antenna proximate to the imaging volume emits a pulse or pulses of radio frequency energy. The radio frequency energy has a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, that shifts the vectors of the nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency receiving antenna tuned to that frequency. The receiving antenna is typically positioned within the imaging volume proximate the patient. Gradient magnetic fields are provided to spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay.
The same antenna may act as the transmitting and receiving antenna. The MR signals detected by the receiving antenna are amplified, digitized and processed by the MRI system. Hydrogen, nitrogen, phosphorous, carbon and sodium are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By correlating the gradient magnetic fields and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient by known mathematical techniques, it is possible to determine the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient's body. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest.
Patients need to lie very still for an extended period of time during an MRI scanning procedure. One type of magnet assembly for performing magnetic resonance imaging on a patient requires that the patient be positioned in a narrow, substantially enclosed gap region within a series of circular superconducting coils spaced along an axis. The walls of the gap may be very close to the patient and may cause claustrophobic reactions in the patient. Certain obese or pregnant patients cannot fit within the gap. In addition, these magnet assemblies may prevent another person, such as a medical attendant or physician, from having easy access to the patient while the patient is within the MRI assembly. Adding to these problems is the significant noise from movement of the magnets that may be generated during the operation of the MRI system. This noise can cause further stress for the patient. It is difficult for those patients who are uncomfortable within the narrow, noisy, claustrophobic gap region to lie still enough for accurate MRI images to be developed. Sedation is often required.
“Open” type MRI assemblies have been developed which have large gap regions for receiving a patient. Open MRI assemblies are described in U.S. Pat. No. 6,201,394 B1, issued Mar. 13, 2001, U.S. Pat. No. 6,023,165, issued Feb. 8, 2000 and U.S. Pat. No. 6,414,490 B1, issued on Jul. 2, 2002, for example, which are incorporated by reference herein, in their entireties. The patient has unobstructed side-to-side views and there is room in the gap for patients to extend their arms, which helps them to relax. Claustrophobic reactions are decreased and it is easier for the patient to lie still without sedation. Obese and pregnant patients can be more easily accommodated, as well. The patient is also easily accessible by a technician or a doctor, which assists in positioning the patient. This patient accessibility is also advantageous in case of emergency.
Despite these advances in MRI assembly design, it has been found that patients still experience claustrophobia and have difficulty remaining motionless due to the surroundings. Since any motion of the patient interferes with the image clarity, there still remains a need in the art to calm a patient during the MRI procedure.